Serving for influencing process variables are field devices in the form of actuators, which e.g., as valves, influence the flow of a liquid in a section of pipeline, or, as pumps, influence level in a container.
A large number of such field devices are manufactured and sold by the firm, Endress+Hauser.
As a rule, field devices in modern fabrication plants are connected via communication networks (Profibus, Fieldbus Foundation, etc.) with superordinated units (e.g. control systems, control units). These superordinated units serve for process control, process visualization, process monitoring, as well as for start-up and servicing of the field devices.
Designated as field devices are, in general, also units directly connected to a fieldbus and serving for communication with the superordinated units (e.g. remote I/Os, gateways, linking devices).
Fieldbus systems can be integrated into enterprise networks working on an Ethernet basis. In this way, process and field device data can be accessed from different regions of an enterprise.
For worldwide communications, the enterprise networks can also be connected with public networks, such as the Internet.
For the servicing of field devices, servicing programs are needed (e.g. FieldCare of Endress+Hauser; Pactware; AMS of Emerson; Simatic PDM of Siemens). Serving for plant control and monitoring are control system applications (e.g. Simatic PCS7 of Siemens; Symphony of ABB; DeltaV of Emerson).
For a plant operator, a high plant availability is of enormous importance. Longer plant downtimes are to be avoided as much as possible, since, most often, significant disadvantages, among others, financial losses, are associated therewith.
The loss of a single field device on a fieldbus system can lead to shutdown of an entire plant. As a rule, the relevant, malfunctioning field device must be replaced. Therefore, device replacement should be capable of being performed as rapidly as possible. A few field device manufacturers offer field devices, which have a removable memory module, in which device parameters important for device functionality are stored, device parameters such as the units of the measured values, alarm limits, TAG-name of the measuring point, etc. In the case of a device replacement, this memory module can easily be inserted into the replacement device. Therefore, the replacement device can be brought quickly into operation, so that the plant can again run.
In the case of the fieldbus system, Fieldbus Foundation, field devices also assume control functions with reference to control loops, this being referred to as “Control in the Field”. These control functions are implemented via so-called “function blocks”. An example of such a function block is a PID block, which can communicate via the fieldbus with other function blocks.
In the case of field devices, which have function blocks, a device replacement is significantly more complicated. As a rule, a device replacement in this case requires the presence of a service technician at the field device and a plant operator monitoring the plant from a control location, e.g. a control room. Generally, a device replacement in the case of such fieldbus systems requires considerably more know-how. Frequently, a device malfunction will occur at an unfavorable time, such as in the night, or on a weekend, when personnel properly schooled for a device replacement are not available.
In many cases, for this reason, plants will even be designed from the beginning to be fully redundant, a practice which is, of course, associated with significant costs.